Various liquid developers for electrostatic photography have hitherto been known, for example, a liquid developer as disclosed in Metcalfe et al., U.S. Pat. No. 2,907,674. Generally, the liquid developer is prepared by mechanically dispersing a pigment or dye such as carbon black, phthalocyanine blue or nigrosine, and a resin such as an alkyd resin, an acrylic resin, rosine, a synthetic rubber, in a solvent having a high electric insulating property using a ball mill, an attritor or a homogenizer, and further adding thereto a charge controlling agent such as a metal soap, lecithin, linseed oil, and a higher fatty acid.
However, the liquid developers obtained by the above-described method generally have problems such as occurrence of precipitates and poor dispersion stability and charge stability due to broad distribution of particle size of the developer. In order to improve the dispersion stability, a liquid developer prepared by using a graft copolymer containing a unit formed from a macromonomer having a molecular weight from 1.times.10.sup.3 to 2.times.10.sup.4 as a dispersion stabilizing resin is disclosed in U.S. Pat. No. 5,112,718. Further, when the liquid developer is used for development of printing plates and for forming a toner image which is used as a resist layer, most of the conventional liquid developers are not satisfactory in the resolving power, resistivity and printing durability.
The formation of the toner image which is used as a resist layer from a liquid developer for electrostatic photography is described hereinafter in detail.
Conventional printing plate materials (printing plates precursors) which utilize electrophotography include zinc oxide-resin dispersion system offset printing materials as described in JP-B-47-47610 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-48-40002, JP-B-48-18325, JP-B-51-15766 and JP-B-51-25761. These printing plate materials are used after a toner image is formed by an electrophotographic process and the non-image areas are wetted with a desensitizing solution (e.g., an aqueous acid solution containing a ferrocyanate or a ferricyanate) to desensitize the areas. The thus processed offset printing plates have a printing durability of about 5,000 to 10,000 prints and are unsuitable for more printing. When the compositions of these plates are designed so as to be suitable for the desensitization processing, there are disadvantages that electrostatic characteristics are deteriorated and image quality becomes poor.
Many organic photoconductive material-resin system printing plate materials are known. Examples of such printing plate materials include those described in JP-B-37-17162, JP-B-38-7758, JP-B-46-39405, JP-A-52-24375 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-B-2-46944. In these printing plates, a styrene-maleic anhydride copolymer, a vinyl acetate-crotonic acid copolymer, a vinyl acetate-maleic anhydride copolymer or a phenolic resin is used as a binder for organic photoconductive materials, said copolymers being soluble in alkalis and/or alcohols. The copolymer together with an organic photoconductive compound is coated on an electrically conductive metallic substrate such as an aluminum sheet to form a sensitive material. The material is subjected to a corona discharge treatment, an exposure treatment and a toner development processing to form a toner image. Non-image areas other than the toner image areas are removed by etching with an aqueous alkaline etching solution, whereby a printing plate can be made wherein the surface of the hydrophilic metallic substrate corresponding to the non-image areas is exposed by etching. As the organic photoconductive material-resin system printing plates according to this system, printing plates which are available under trade name of Elefasol from Curry Co., are put to practical use. However, the Elefasol system is a system wherein a toner image is formed with a dry developer. Even when fine toner grains are used as the dry developer, printing plates giving images having poor resolving power of only about 3 to 5 lines/mm can be obtained.
On the other hand, when a toner image is formed by using a liquid developer, there can be obtained an image having a resolving power of about 15 to 50 lines/mm.
When the liquid developer is used, a toner image excellent in resolving power can be obtained and a sharp image can be obtained. However, there are disadvantages that the thickness of the toner image is considerably thinner than that of the dry system image and the toner image is inferior to the dry system image in the property as a resist in etching solutions and as a result, the resulting printing plate has poor printing durability.
Generally, it is required that liquid developers for printing plates have such characteristics that the developers are excellent in dispersion stability, redispersibility and fixability in addition to the excellent property as a resist (resistivity) to the etching solutions. Many liquid developers for printing plates have been conventionally developed and proposed. However, the fact is that there is not proposed any liquid developer which is considered to have all of the desired characteristics with regard to resolving power, dispersion stability, redispersibility, fixability and the property as a resist.